Display devices equipped with display means such as liquid crystal members and LED members are in wide use as, for example, small and/or portable display devices such as electronic notebooks, laptop personal computers, tablet PCs, and smartphones. For protection of these display devices, cover glass is provided on the surface of these display devices.
Display devices, particularly portable display devices have a relatively higher chance of being accidentally dropped by a user during use or during carrying. This has created a need for a cover glass having high-strength that can avoid damages on the cover glass even when the display device is dropped.
The recent trend for higher-resolution display devices also requires the cover glass to provide high visibility for display image so that the high resolution properties will not be impaired.
One way of increasing the strength of a cover glass is to carry out a chemical toughening treatment on the cover glass.
Commonly known soda lime silicate glass involves problems in that it has slow ion exchange rates and thus is not sufficient in terms of the depth of the surface compressional stress layer (DOL) formed by chemical toughening treatment, and that this type of glass also has high brittleness and thus cracking tends to occur during handling.
In order to solve these problems, the present applicant has provided the glass for chemical toughening described in Patent Document 1. This glass for chemical toughening has a high ion exchange rate, and thus the depth of the surface compressional stress layer (DOL) formed by chemical toughening treatment is sufficient. The glass also has low brittleness, and cracking is unlikely to occur during handling.
However, the glass for chemical toughening described in Patent Document 1 involves problems in that it has poor productivity because of high melt viscosity, that it has a low glass transition point (Tg) causing stress relaxation during the chemical toughening treatment, and thus a high surface compressional stress (CS) cannot be attained.